1. Field of the Invention
This invention relates to optical glass of high refractive index and high dispersion capability.
2. Description of the Prior Art
Heretofore, optical glasses of high refractive index and high dispersion have included those which are within the range of the so-called biflint glass or lanthanum biflint glass and which contain as a main component glass mesh forming oxide such as silicic acid (SiO.sub.2) or boracic acid (B.sub.2 O.sub.3), glass mesh modifying oxide such as alkali metal oxide, alkaline earth metal oxide, zinc oxide (ZnO), lead oxide (PbO), titanium oxide (TiO.sub.2), lanthanum oxide (La.sub.2 O.sub.3), zirconium oxide (ZrO.sub.2), tantalum oxide (Ta.sub.2 O.sub.5) or niobium oxide (Nb.sub.2 O.sub.5), or intermediate oxide such as aluminum oxide (Al.sub.2 O.sub.3). These optical glasses have been widely used as the material of the elements forming optical instruments, but they are not always satisfactory in transmissivity to light and chemical durability. Such tendency is particularly strong in those materials which have high refractive index and which contain less silicic acid and more lead oxide. Recently, along with the wide spread use of color photography, the requirement for the transmissivity to light of optical glass has become exacting, and the yellowish color peculiar to the optical glasses which are within the above range has been regarded as a greater disadvantage than before. Also, the advantage of the optical glass polishing technique has often exposed the optical glasses to severe polishing conditions and along with the spread of a multilayer coating, the presence of corrosive degenerating layers in the surface of the glass has become impermissible, and accordingly, chemical durability of the glass has become a more important factor than heretofore.